When comparing Jam vs Ender, the Slant community recommends Ender for most people. In the question“What are the best front-end package managers?”Ender is ranked 4th while Jam is ranked 6th. The most important reason people chose Ender is:

Ender allows you to expose your packages into the $ namespace, which allows you to effectively create a customized jQuery. You can provide it with the selector engine, a DOM manipulation library, and an event library of your choice, as well as extend it with additional utilities. By using micro-libraries instead of jQuery, you can get smaller file sizes and more customized utility with more flexibility.
Ender accomplishes this by allowing a package to describe its functionality with an optional javascript "bridge" file that allows it to integrate with Ender.

Pro

Provides the best AMD compatibility allowing for better asynchronous loading

Pro

Lets you bundle packages into a custom jQuery like object

Ender allows you to expose your packages into the $ namespace, which allows you to effectively create a customized jQuery. You can provide it with the selector engine, a DOM manipulation library, and an event library of your choice, as well as extend it with additional utilities. By using micro-libraries instead of jQuery, you can get smaller file sizes and more customized utility with more flexibility.

Ender accomplishes this by allowing a package to describe its functionality with an optional javascript "bridge" file that allows it to integrate with Ender.

Pro

Add dependencies and bundle into a package in one utility

As opposed to Browserify, Ender has the package management portion and bundler as one. Browserify is only a bundler, meaning you need to manage packages with npm as well. Using npm is good if you have frontend and backend dependencies, as you need to use npm for the backend anyway, but not as nice if you only need to create a package for the frontend.

Pro

Uses the npm repository

Because Ender uses the npm repository, it gains access to its huge community and plenty of library support.

Pro

Supports require through CommonJS

Ender is able to allow you to use use Node.js-style requires, as it gets its packages from npm, which are all required to support CommonJS.

Pro

Small lightweight modules

Ender encourages the use of smaller modules through a jQuery like object. Smaller projects benefit from being developed in parallel which can lead to faster release cycles for a single component, compared to a monolithic project like jQuery. You can also find more diverse functionality by using smaller projects.

By just using the components you need leads to a smaller file size and faster load speed. It also allows you to be more opinionated about which components you want to use, so you don't have to use an API you don't agree with just because it needs to provide backwards compatibility.

Cons

Con

Forced AMD compatibility means fewer libraries

AMD is currently not as popular as CommonJS modules, which means if a library isn't supported, you'll have to deal with it yourself.

Con

Utilizing a registry not made for the browser

Ender is another attempt at hijacking and using npm for downloading browser dependencies. But in doing so, it's using a registry and module specifications not made for the browser. This means that there will have to be some extra work to get them to work properly.